You can't unless you have an A4 device with an exploitable bootrom or SHSH blobs saved. You can't "downgrade" in a browser like that, it's completely impossible.
It's a completely different process to a restore. Star used a PDF exploit to work it's way up to a kernel exploit allowing for the patching out of various protections which allowed for a jailbreak. You could not have used this exploit kit to restore an iPhone in the way this video claims to.
You can't unless you have an A4 device with an exploitable bootrom or SHSH blobs saved. You can't "downgrade" in a browser like that, it's completely impossible.